In order to establish communications in a wireless system a User Equipment (UE) must first synchronize with a base station. Once synchronization is established, the substantive communication and/or data transfer may occur such that a wireless telephone call may be conducted.
The 3rd Generation Partnership Project (3GPP) in, for example, 3GPP TS 25.221 v5.2.0, 3GPP TS 25.223v5.1.0 and 3GPP TS 25.224 v5.2.0, specifies communication systems that employ a relatively high chip rate of 3.84 Mcps or, optionally, a relatively low chip rate of 1.28 Mcps. In the specified high rate option, a User Equipment (UE) searches for a known Primary Synchronization Code (PSC) and then identifies one of a number of different groups of secondary synchronization codes. However, there is no single PSC in the low chip rate option. A UE must search for a downlink synchronization code SYNC-DL which may be one of 32 different 64 element sequences.
FIG. 1 illustrates the time frame structure for the 1.28 Mcps low chip rate option of a wireless system as currently specified by 3GPP. Ten (10) ms frames are divided into two sub-frames of five (5) ms each. Each sub-frame includes seven (7) timeslots and a separate area for uplink and downlink synchronization (SYNC) signals. Each Timeslot 0-6 is configured to receive communication data symbols and an identifying midamble code. Timeslot 0 is always a downlink (DL) slot. Timeslot 1 is always an uplink (UL) slot. Timeslots 2-6 are configurable for either UL or DL usage.
Between Timeslot 0 and Timeslot 1, there exists a ninety-six (96) chip long Downlink Pilot Timeslot (DwPTS), a ninety-six (96) chip long guard period (GP) and a one-hundred sixty (160) chip long uplink pilot timeslot (UpPTS). Within the DwPTS there is a thirty-two (32) chip long guard period and a 64 chip Synchronous (SYNC-DL) code section. In addition, every two (2) frames (four sub-frames) defines a 20 ms superframe.
In the current 3GPP system specification, there are thirty-two (32) SYNC-DL codes, each having sixty four (64) elements. Each SYNC-DL code points to four basic midamble codes (of length 128) so that there are total of 128 basic midamble codes. In addition, each timeslot's midamble code (of length 144) is generated from a basic midamble code (of length 128). From each basic midamble code, up to 16 timeslot midamble codes of length 144 can be generated.
Quadrature Phase Shift Keying (QPSK) modulation is used on the SYNC-DL codes. In each sub-frame, the midamble code in the DL Timeslot 0 provides a QSPK phase reference of the SYNC-DL code in the DwPTS. Accordingly, once the midamble code of Timeslot 0 is determined the QPSK modulation of a SYNC-DL code in the DwPTS of the sub-frame can be ascertained. The timing of the superframe (SFT) is indicated by a specified sequence of the Quadrature Phase Shift Keying (QPSK) modulation on the SYNC-DL code over a specified number of sequential sub-frames.
An objective of synchronization is to be able to receive data of a broadcast channel (BCH) which is carried by a Primary Common Control Physical Channel (P-CCPCH) in Timeslots 0 of a superframe. Presently, two different sequences of SYNC-DL code modulation are specified for four sequential DwPTS in a superframe, 3GPP TS 25.223 v5.1.0 Sec. 9.1.1. A first sequence, S1, indicates that there is a P-CCPCH carrying a BCH in the next superframe; a second sequence, S2, indicates that there is no such P-CCPCH in the next superframe. Where sequence S1 of the modulation of the SYNC-DL codes of a superframe is found, the data from the BCH can be read from the P-CCPCH of the next superframe.
Annex D of 3GPP TS 25.224 V5.2.0 suggests a four step procedure for UE determination of synchronization which is graphically depicted in FIG. 2. The first step requires the system to search through the 32 codes to determine which SYNC-DL code is being received and to determine the code timing, i.e. where in the stream of received data the DwPLTSs carrying the SYNC-DL code are located as a reference with respect to the system time frame structure. Step two of the process determines which one of the four basic midamble codes, as indicated by the SYNC-DL, is used. This is completed by processing the midamble section of Timeslot 0 (P-CCPCH). Since the midamble and the scrambling code are tied together on a one-to-one correlation, once midamble is known, the scrambling code is also known. If this step fails, the first step is repeated.
During step three, the process determines the phases of the QPSK modulation that is on the SYNC code over multiple sub-frames and from this the superframe timing (SFT) is determined. At step four, the complete broadcast channel (BCH) information is read by the UE.
In view of the provision for the 1.28 Mcps option, there is a need for a UE which has a receiver capable of conducting synchronization in an efficient manner without undo hardware cost.